The thinning of pipe walls in nuclear power plants recently has been recognized as a serious industrywide problem. More specifically, it has been discovered that carbon steel pipe, which carries wet steam in nuclear power plants, is especially susceptible to erosion and corrosion. In some instances, pipe wall thicknesses have been reduced at a rate of 1.0 to 1.5 millimeters per year, yet many plants do not have in place an inspection program for examining the thickness of the pipe walls. Such pipe wall degradation rates, at best, are severe enough to cause pipe failures resulting in costly repairs and plant downtime. At worst, plant workers could be injured or killed. Thus, it can be seen that pipe wall thickness inspections are necessary to detect and periodically monitor the severity of such degradation, and in fact now are required of all nuclear power plant licensees by the United States Nuclear Regulatory Commission.
However, virtually all carbon steel piping carrying wet steam in nuclear power plants is insulated to minimize heat loss. Thus, all known current inspection techniques, such as the ultrasonic method, require costly insulation removal and site preparation in order to measure the pipe wall degradation. If the insulation is asbestos, as is the case in many plants, inspection can be further complicated due to the health hazards associated therewith.
The known prior art includes various methods and apparatus which utilize radiation for measuring the wall thickness of a pipe or the thickness of other objects such as a sheet or strip of material,etc. Such known prior art is disclosed in U.S. Pat. Nos. 2,859,349; 3,808,437; 4,088,886; 4,182,954; 4,542,297; and 4,695,729. However, the apparatus, methods and results obtained by the inventions shown therein are completely different than the apparatus, method and objectives achieved by our invention. More specifically, the disclosure of these patents includes a plurality of radiation sources and/or detectors, the necessity of moving the object being measured, elongated sources and/or detectors, obtaining measurements of average thickness, use of a relatively high-level radiation source, etc.
The known prior art also shows methods and apparatus for weighing material using radiation. Such known methods and apparatus are disclosed in U.S. Pat. Nos. 3,278,747 and 3,489,901. Again, however, such methods and apparatus require movement of the material being measured and utilize multiple and elongated sources and detectors.
The closest known prior art to our present invention is shown in U.S. Pat. No. 3,474,160. However, the method and apparatus shown and described in this patent is considerably different than our invention. First, the source and detector of the apparatus disclosed in this '160 patent continuously move back and forth across the sheet being measured to take continuous measurements of the sheet thickness. In contrast, the source and detector of the present invention move in discrete linear steps for scanning a pipe to take discrete measurements of the pipe wall thickness. In addition, the source and detector shown in the '160 patent are mounted on traversing rods which extend between mounting brackets, with the source and detector being directly driven by a traversing motor. In comparison, the source and detector of the present invention each is mounted on its own frame assembly. Elongated rotatable shafts of the frame assemblies are operatively connected to the source and detector. A motor rotates the shafts and means are utilized for translating the rotational motion of the shafts into the linear motion of the source and detector.
Therefore, the need exists for a method and apparatus for measuring the wall thickness of insulated pipe of various shapes, sizes and locations, without requiring costly insulation removal and site preparation.